The Big Bang happened. The evidence is clear, ranging from the underpinnings of Albert Einstein’s general theory of relativity, to the detection of the cosmic microwave background by Arno Penzias and Robert Wilson in the 1960s, to the confirmation of ripples in the fabric of ancient space-time from the Cosmic Background Explorer (COBE) satellite in 1992.
When the Big Bang occurred, matter, energy, space, and time were all formed, and the universe was infinitely dense and incredibly hot. The universe cooled rapidly as it blew outward, however, and by 10–35 second after the Big Bang, the epoch of inflation occurred. During this wild period, cosmic strings, monopoles, and other exotic species likely came to be.
After inflating, the universe slowed down its expansion rate but continued to grow, as it does still. It also cooled significantly, allowing for the formation of matter — first neutrinos, electrons, quarks, and photons, followed by protons and neutrons. Likewise, antiparticles were produced in abundance, carrying the opposite charge of their corresponding particles (positrons along with electrons, for example).
As time went on and particles’ rest-mass energy was greater than the thermal energy of the universe, many were annihilated with their partners, producing gamma rays in the process. As more time crept by, these annihilations left an excess of ordinary matter over antimatter.
The relic radiation of the Big Bang decoupled (picture heavy traffic suddenly clearing) nearly 400,000 years later, creating the resonant echo of radiation observed by Penzias and Wilson with their radio telescope. This decoupling moment witnessed the universe changing from opaque to transparent. Matter and radiation were finally separate.
For the full story on how the Big Bang happened, see here: https://www.astronomy.com/science/how-did-the-big-bang-happen/
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